docs: def export_to_xml(self, block, xmlfile):
          """
          Export the block to XML, writing the XML to `xmlfile`.
          """
          root = etree.Element("unknown_root", nsmap=XML_NAMESPACES)
          tree = etree.ElementTree(root)
          block.add_xml_to_node(root)
          # write asides as children
          for aside in self.get_asides(block):
              if aside.needs_serialization():
                  aside_node = etree.Element("unknown_root", nsmap=XML_NAMESPACES)
                  aside.add_xml_to_node(aside_node)
                  block.append(aside_node)
          tree.write(xmlfile, xml_declaration=True, pretty_print=True, encoding='utf-8')